1. Field of the Invention
The present invention relates to a technology for reproducing information recorded on an optical disk, and more specifically to an apparatus and a method for reproducing information recorded on an optical disk for high-density recording.
2. Description of the Prior Art
There are three types of optical disks commonly used at present. They are: (1) reproducable-only optical disks; (2) once-writable optical disks; and (3) freely writable optical disks. The reproducable-only optical disks can be CD, CD-ROM, DVD-ROM, DVD-Video, etc. The once-writable optical disks can be CD-R, DVD-R, etc. The freely writable optical disks can be MO, PD, DVD-RAM, etc. Among the above described optical disks, reproducable-only optical disks are made of a substrate of polycarbonate, etc. in which holes called xe2x80x98pitsxe2x80x99 are made to provide information to be recorded. A high-reflective film of Al, AlTi, etc. is coated on the substrate. When recorded information is reproduced, a laser beam, etc. is emitted and forms an optical spot on the above described pits. By detecting the laser beam reflected on the optical spot, the optical spot causes optical interference at a point where a pit exists, thereby reducing the amount of the reflected light. Since the intensity of a light depends on the existence and non-existence of a pit and detected as a signal, it can be retrieved as information. An optical detector for detecting a reflected light from an optical disk is disclosed by Japanese Patent Laid-Open No. 5-101414. According to this technology as shown in FIG. 9, the optical disk is equally divided into four sections A, B, C and D by an axis in the proceeding direction (along the circumference of an optical disk) of an optical spot on the optical disk, and by an axis vertical to the axis in the proceeding direction. A sum (A+B+C+D) of the outputs of the quarters A, B, C, and D of the optical detector forms a reproducing signal. This signal is referred to as a sum signal in this specification. A track servo signal for use in leading the optical spot along the above described pit when the information recorded on the optical disk is reproduced can be a signal obtained by (A+B)xe2x88x92(C+D) or (C+D) (A+B) in, for example, DVD-ROM, DVD-Video, etc.
A once-writable disk is made of a substrate of polycarbonate on which an organic chromogenic film, etc. is coated. When information is recorded, a high-level output optical beam is emitted to a point at which the information is recorded, and a hole is made in the organic chromogenic film as a predetermined mark, thereby generating an object corresponding to a pit of the read-only optical disk. Otherwise, information can be reproduced as in the reproducable-only optical disk. The pit and the corresponding mark can be data division signal. In the once-writable optical disk, the optical detector shown in FIG. 9 is used, and a reproducing signal is formed by a sum of the outputs from the quarters A through D of the optical detector (sum signal). On the other hand, as a track servo signal, a difference signal obtained by the difference between the right and left halves of the optical disk in the proceeding direction of the optical spot is used such as (A+C)xe2x88x92(B +D) or (B+D)xe2x88x92(A+C). On the once-writable optical disk, a pit similar to that of the reproducable-only optical disk exists on the substrate. The pit portion is referred to as an ID division. The ID division can be read by a sum signal when information is reproduced, or can also be read by a difference signal.
Furthermore, a freely writable optical disk is made of a substrate of polycarbonate coated with plural layers of thin films containing a recording film. The recording film is a magnetic substance such as TbTeCo, etc. in an MO (magneto-optic disk), and is made of chalcogenide phase change material such as GeSbTe in a PD and DVD-RAM.
An MO detects a signal by a method of detecting a change in the deflection direction of an incident light, and is basically different from the above described method of detecting a signal according to the above described sum and difference signals. When information is recorded on an optical disk, a high-level output optical beam is emitted to a point at which the information is to be recorded, and a predetermined mark is formed by reversing the magnetization of the magnetic substance. The information is recorded on the recording film as a data division signal. However, in an MO, there is also a pit referred to as an ID division on the substrate, and the signal is read by a sum signal.
On the other hand, an optical disk made of a phase change material, for example, PD, DVD-RAM, etc. detects a signal by detecting the difference in intensity of optical reflection between a crystal division and an amorphous division. That is, the detection is carried out by using a sum signal. When information is recorded, a high-level output optical beam is emitted to a point at which the information is to be recorded, the phase change film is set amorphous, and a mark is formed. The information recorded on the recording film is referred to as a data division signal.
A pit referred to as an ID division exists on a substrate of an optical disk made of the above described phase change material. In a PD, an ID signal (ID information) is detected by using a sum signal. In DVD-RAM, an ID signal (ID information) is detected by using a difference signal. In the above described optical disks, a difference signal obtained by (A+C)xe2x88x92(B+D) or (B+D)xe2x88x92(A+C), etc. is used as a track servo signal.
As described above, a signal relating to information recorded on an optical disk can be often detected by using a sum signal or a difference signal in an optical disk. Generally, in case of the difference signal reproducing system, an optical spot does not pass through the central part of a pit or a recording signal as shown in FIG. 10(a), but passes through the edge part of the pit or the recording signal. On the other hand, in case of the sum signal reproducing system, an optical spot often passes through the central part of a pit or a recording signal as shown in FIG. 10(b). However, even in case of reproducing a sum signal, information can also be reproduced in principle in the form as shown in FIG. 10(a).
The merit of the sum signal reproducing system is to obtain a stable read signal output without a large difference in a reproducing signal even when the optical axis of an optical spot cannot be vertical to the recording surface of an optical disk because, for example, the optical disk becomes tilted.
However, since both diffracted light in the proceeding direction of an optical spot and diffracted light vertical to the proceeding direction of the optical spot are detected, a reproducing signal reflects the characteristics of both diffracted lights. As a result, as shown in FIG. 11, there arises the problem that the center position (center level) of signal amplitude is shifted (fluctuates) depending on the length of the mark and the pit in the proceeding direction of the optical spot.
Thus, shifting the center position of signal amplitude depending on the length of a mark or a pit means that there arise a mark or a pit having a correctly detected length and a mark or a pit having an incorrectly detected length depending on how a slice level is determined after slicing a detection signal at a certain level (threshold) to generate a binary signal indicating 0 or 1. As a result, the jitter (fluctuation of the edge of a signal) rises, thereby increasing miss-detection of data. This phenomenon conspicuously appears when a pit formed on a substrate is detected.
On the other hand, in the difference signal system, the difference between the output on the right and the output on the left in the proceeding direction of an optical spot is computed. Therefore, the left and right diffracted lights in the direction vertical to the proceeding direction of the optical spot are offset by each other. As a result, only the effect of the diffracted light in the proceeding direction of the optical spot works. Accordingly, a detected signal is subject to only the effect of a diffracted light in one direction, thereby indicating a simple characteristic. Therefore, as shown in FIG. 11, the center positions of the signal amplitude of all lengths of marks and pits match. Therefore, the slice level of a detection signal when it is binarized is uniquely determined, thereby correctly detecting the edges of all lengths of marks or pits.
However, a difference signal is different from a sum signal as follows. That is, when an optical disk is tilted, especially when it is tilted to right or left about the surface vertical to the proceeding direction of the optical spot, the left-right balance of the reflected light of the optical spot cannot be maintained. Therefore, the offset between the left and right diffracted lights becomes incomplete, thereby causing a large change in a reproducing signal and the problem that a stable output can hardly be obtained.
The present invention aims at providing a method and an apparatus for reproducing information stored on an optical disk by effectively detecting a reproducing signal with a high margin.
The feature of the method of reproducing information on an optical disk according to the present invention resides in reproducing information recorded on an optical disk according to a signal obtained by adding up optical outputs from two optical detectors positioned opposite each other in the proceeding direction of an optical spot emitted to the optical disk. In this case, the optical outputs of two other optical detectors positioned opposite each other in the direction vertical to the proceeding direction of the optical spot are added up. The sum is multiplied by a coefficient larger than 0 and smaller than 1. Then, the resultant output is added to the above described sum signal to preferably reproduce information. Furthermore, the above described two optical detectors are divided into two sections in the direction vertical to the proceeding direction of the optical spot, and the other two optical detectors are divided into two sections in the proceeding direction of the optical spot so that the information can be reproduced by track-controlling the optical spot relating to the optical disk using each optical output from each of the divided optical detectors.
In addition, the optical disk reproducing apparatus according to the present invention includes a unit for emitting an optical spot to an optical disk, an optical detector for detecting a light transmitted through or reflected by the optical disk, and an operations unit for computing an output from the optical detector to obtain an information reproducing signal. With the configuration, the optical detector includes the first and the second optical detectors positioned opposite each other in the proceeding direction of the optical spot on the optical disk. The operations unit includes a unit for obtaining the first sum signal obtained by adding up the optical outputs from the first and the second optical detectors. Furthermore, according to the present invention, the optical detector includes the third and the fourth optical detectors positioned opposite each other in the direction vertical to the proceeding direction of the optical spot. The operations unit includes: a unit for obtaining the second sum signal obtained by adding up the optical outputs from the third and the fourth optical detectors; and a unit for multiplying the second sum signal by a coefficient larger than 0 and smaller than 1 and adding the obtained product to the first sum signal. Furthermore, it is desired that the first and the second optical detectors are divided into two portions in the direction vertical to the proceeding direction of the optical spot, and that the third and the fourth optical detectors are divided into two portions in the direction vertical to the proceeding direction of the optical spot.
According to the present invention, the above described two former optical detectors receive diffracted lights in the proceeding direction of the optical spot, and the above described two latter optical detectors receive diffracted lights in the direction vertical to the proceeding direction of the optical spot. A diffracted light in the direction vertical to the proceeding direction of the optical spot depends on the width of a mark or a pit, and a diffracted light in the proceeding direction of the optical spot depends on the length of a mark or a pit. However, with the above described configuration, a diffracted light in the proceeding direction of the optical spot can be mainly detected by using as a reproducing signal a sum obtained by multiplying a sum of the optical outputs from the former two optical detectors or a sum of the optical outputs from the later two optical detectors by a coefficient a (0 less than xcex1 less than 1). In case of a sum signal, the diffracted light contains the characteristics about the edge of a mark or a pit. Therefore, there arises no problems that the center position of signal amplitude may be shifted depending on the length of the mark or the pit in the proceeding direction of the optical spot. As a result, a high margin specific to a sum signal can be obtained without the problem that the center position of signal amplitude may be shifted depending on the length of a mark or a pit in the proceeding direction of the optical spot.